Plastic containers are commonly used for the packaging of food and for a wide variety of other items wherein a plastic lid is bonded to the container e.g. by the application of heat.
In order to package products, in particular food products, vacuum packaging have been developed in the past.
Among the known vacuum packaging processes, vacuum skin packaging is commonly employed for packaging food products such as fresh and frozen meat and fish, cheese, processed meat, ready meals and the like. Vacuum skin packaging is described for instance in FR 1 258 357, FR 1 286 018, AU 3 491 504, US RE 30 009, U.S. Pat. No. 3,574,642, U.S. Pat. No. 3,681,092, U.S. Pat. No. 3,713,849, U.S. Pat. No. 4,055,672, and U.S. Pat. No. 5,346,735.
Vacuum skin packaging is basically a thermoforming process. In particular, the product is typically placed on a rigid or semi-rigid support (such as a tray, a bowl or a cup). The support with the product placed thereon is put in a vacuum chamber, where a film of thermoplastic material, held by vacuum in a position above the product placed on the support, is heated to soften it. The space between the support and the film is then evacuated and finally vacuum above the film is released to cause the film to drape down all around the product and seal to the surface of the support not covered by the product, thus forming a tight skin around the product and on the support.
US 2005/0257501 discloses a machine for packaging a product arranged in a tray. The machine has a lower tool for supporting the tray and an upper tool with a cutting device. During operation, the film is clamped along an edge surrounding the tray and is deformed by the upper tool in a direction extending away the product. The space surrounding the product is then evacuated, the film and the edge of the tray are sealed and the film is then cut by the cutting device.
As to the machines disclosed by US 2007/0022717 and US 2005/0257501, the film is cut to the size of the tray within the chamber formed by the upper tool and the lower tool, by means of the cutting devices provided on the upper tool. First of all, this disadvantageously requires providing a rather complex and bulky upper tool. Besides, this disadvantageously requires providing an excess film with respect to the size of the support, which excess film is cut from the package and scrapped during or at the end of the packaging process. Indeed, the film is in the form of a continuous sheet wound on a roll (as shown e.g. in FIG. 3 of US 2005/0257501). Therefore, an excess film is required to allow the film to be pulled from the roll and to be held in place above the supported product. Further, in US 2007/0022717 more than one product loaded support (namely two) is packaged at each cycle, so that an excess film is also present between adjacent supports.
DE102006022418 discloses a an apparatus where a sealing film is cut to size and fixed to the upper rim of a tray inside the sealing station. This solution requires a quite complex design of the sealing station in order to host and operate the cutting blades. Furthermore, an excess film is required to allow sealing and cutting of the film. Furthermore described in this reference provides for a little tube operating through a valve on the tray side wall or through a gap between the sealing film and the tray upper rim.
WO2011/012652 shows an apparatus for packaging a product in a tray. The machine comprises a first film transfer plate configured for holding a film sheet, heating the film sheet, bringing the film sheet to a position above a tray with the product arranged thereon and air tightly fixing the film sheet to the tray. A second film transfer plate is also present. As for the first film transfer plate also the second film transfer plate is configured for holding a film sheet, heating the film sheet, bringing the film sheet to a position above a tray with the product arranged thereon and air tightly fixing the film sheet to the tray. During a first operating step of the machine, the first film transfer plate holds a first film sheet and heats the first film sheet, while the second film transfer plate releases a second film sheet thereby allowing the second sheet to be drawn into a first tray; and during a second operating step of the machine, the second film transfer plate holds a third film sheet and heats the third film sheet, while the first film transfer plate releases the first film sheet thereby allowing the first film sheet to be drawn into a second tray. The machine further comprises a rotating cylinder suitable for rotating about its axis X, the first film transfer plate and the second film transfer plate being connected to the rotating cylinder so that, when the rotating cylinder rotates about its axis X, the positions of the first film transfer plate and the second film transfer plate are exchanged. A vacuum arrangement allows removing air from within the tray underneath the film sheet (positioned either by the first or by the second film transfer plate) through the hole or holes present in the tray. The film transfer plates are configured to release the film sheet thereby allowing the film sheet to be drawn into the tray while the vacuum arrangement is removing air from within the tray. This solution has allowed significant savings in term of film material, but requires cutting the film to size before the film is sealed to the tray.
Besides the above solutions, packaging apparatus have been developed wherein trays are in-line formed from a continuous bottom web of plastic material. The in-line formed trays, after being filled with an appropriate product, are sealed by a continuous top film. The plurality of sealed trays are then longitudinally and, if necessary transversally, separated by cutters located down stream the sealing station. In this kind of apparatus, the bottom web is perforated in correspondence of the longitudinal side edges thereof. This is done to create apertures which allow air removal from the trays in correspondence of the sealing station, before fixing the top film to the trays. Furthermore, chains provided with grip means guide the bottom web all along its path from formation of the trays up to the final separation. The perforated side edges are removed from the trays and cause a non negligible waste of web and film material.
EP0293794 B1 discloses a packaging apparatus where a continuous sheet of plastic packaging material conveyed by a chain is formed into cup-shaped containers loaded with product and arranged in two side-by-side rows. Evacuation openings are cut through the plastic between successive container pairs: the cuts are located in the middle of the transversal rim connecting two consecutive trays and extend in the cross machine direction. A second continuous web of plastic packaging material is laid down over the first web to cover the filled containers and thereby form packages. An appropriate design of the sealing station allows to withdraw gas from the trays via the openings and to then completely seal the trays. This solution, requires that longitudinally adjacent trays be separated by a sufficiently sized transverse rim, thus causing waste of material. Note that if the tray rims concerned with the openings are not finally trimmed, the packaged try may also present rims having a quite irregular shape.
DE 2161465 show a packaging apparatus where trays are continuously formed from a lower continuous plastic web which is guided to a sealing station. A top sealing film is longitudinally cut and is applied to the rows of trays. The longitudinal cuts allow air withdrawal and, after air withdrawal, are eventually sealed at the packaging assembly. Note that longitudinally cutting the top film before the actual sealing to the tray may compromise the ability to accurately guide the top film and certainly increases complexity of the sealing operation.
In this situation, it is an aim of the present invention to offer a process and an apparatus where trays are formed in-line and at the same time scrap of packaging material is minimized.
A further object is that of offering a process and an apparatus capable of properly guiding the trays and the sealing film such as to reduce if not avoid problems of mis-positioning of the sealing film onto the tray.
Additionally, it is an auxiliary object of the invention conceiving a process and an apparatus which can flexibly operate both for skin packaging and for modified atmosphere packaging.
It is a further aim of the invention that of conceiving a process and an apparatus for packaging products using either heat shrinkable films or non heat shrinkable films.